FXUS66 KPDT 100533 AAA AFDPDT Area Forecast Discussion...UPDATED AVIATION. National Weather Service Pendleton OR 1033 PM PDT Thu May 9 2024 .UPDATE...A quiet weather evening is underway with clear skies aside from some mountain cumulus which should dissipate later on. Winds are beginning to decrease and should be light overnight. High pressure will help keep temperatures warmer tonight than last night - as much as 10 degrees warmer in the Columbia Basin. Current forecast looks good and no changes were made. One mild concern going into this weekend and next week are increased snow melt from the warmer temperatures in those parts of the mountains that still have a snowpack. Several rivers and streams in the mountains are forecast to rise though near all stay well below bankfull. The exception is the Naches River at Cliffdell which is forecast to rise slightly above bankfull Saturday evening and the streamflow data shows the beginnings of a rise in the last few hours. Probabilistic forecasts for the Naches at Cliffdell have been steadily rising from an 88 percent chance of exceeding bankfull in the next 10 days a couple of days ago to a greater than 95 percent chance in the latest forecast. The good news is that the Naches River at Cliffdell is expected to crest at just a few inches above bankfull Monday morning though it is expected to remain barely above bankfull through Thursday. At these levels, there should be few, if any, impacts. Further downstream, the Naches River at Naches is forecast to rise as well but remain below bankfull. However, the probabilistic forecast for the Naches River at Naches does give a 33 percent chance of exceeding bankfull at some time in the next 10 days, so that's another location to be monitored through then. 83 && AVIATION...06z TAFS...Little to no aviation impacts through the TAF period. Latest satellite imagery shows a few swaths of high clouds drifting from the northeast over eastern OR and a few isolated areas of lingering stratocu. High confidence (>80%) in limited sky cover over the basin and lowlands through the TAF period. Elsewhere, diurnal mountain cu development is expected tomorrow afternoon with good agreement seen in HREF and hi-res guidance. Current thinking is areas with the best chance for this will include the higher terrain of central OR, Ochocos, John Day Highlands, and eastern mountains with bases >6 kft and higher. Otherwise, an upper-level ridge aloft with a weak surface pressure pattern will prevail prompting light winds and breezes through the period. && .PREV DISCUSSION... /issued 455 PM PDT Thu May 9 2024/ SHORT TERM...Tonight through Saturday night...A large ridge of high pressure will build over the Pacific Northwest through Saturday. This ridge will flatten a bit through Saturday night. Dry and increasingly warm conditions are expected through the weekend. High temperatures will be 15 to 20 degrees above normal on Saturday, on average across the area. Probabilities of high temperatures >=90 degrees on Saturday range from about 60% in the lower Columbia Basin of Oregon to about 60 to 70% in the Yakima Valley and 70 to 80% in the Columbia Basin of Washington. None of the GFS ensemble members reach >=90 degrees on Saturday for Tri-Cities whereas approximately 50% of the ECMWF ensemble members >= 90 degrees. High temperatures will range from the upper 70s to mid 80s on Friday, then generally in the 80s to around 90 degrees on Saturday. Overnight lows will be in the 40s tonight and the 40s to lower 50s Friday night. LONG TERM...Sunday through Thursday..A shortwave trough will approach the area on Sunday and move across the region Sunday night. The ensemble clusters favor a flat wave compared to the higher amplitude depicted in the deterministic models. Sunday will be the last very warm day with high temperatures in the mid 80s to lower 90s in the lower elevations. Probabilities of 90+ degree high temperatures Sunday are 30-70% across the Columbia Basin and Yakima Valley. A cold front will cross the area Sunday associated with the shortwave and it will become breezy through the Cascade gaps in the afternoon and evening. The winds will continue on Monday along with cooler temperatures (8-10 degrees cooler than Sunday.) The 12Z GFS is forecasting the surface pressure gradient between PDX and GEG to exceed 10 mb on Sunday night and Monday which supports windy west winds in the lower elevations. Probabilistic NBM for the 24 hours ending at 11 PM Monday shows a 50-70% chance of winds exceeding 45 mph in the usual windy locations under strong westerly flow regimes. On Tuesday into early Wednesday the GEFS and ECMWF ensemble build a ridge into the region from offshore with NWLY flow aloft over the area. This will result in dry and warm conditions on Tuesday and possibly Wednesday though the uncertainty in the forecast begins to increase on Wednesday and even more on Thursday. For Thursday the ensemble clusters are split with roughly half the ensemble members favoring a trough solution and the other half favoring a ridge. This is creating significant uncertainty in the temperature forecast for mid week. For example, on Thursday at Pendleton the NBM 25th percentile high temperature is 66 degrees and the 75th percentile is 85 degrees. The GFS and ECMWF ensemble temperatures are cooler than the official forecast so it is possible that temperature will have to be nudged downward in later forecasts. The chance for precipitation appears quite low through the period. Except for occasional POPS 15-20% along the Cascade crest, Northern Blues and Wallowas, the chance for measurable precipitation elsewhere is 10% or less. 78 && .PRELIMINARY POINT TEMPS/POPS... PDT 47 79 51 84 / 0 0 0 0 ALW 50 82 54 86 / 0 0 0 0 PSC 52 86 55 91 / 0 0 0 0 YKM 48 85 51 90 / 0 0 0 0 HRI 49 84 53 90 / 0 0 0 0 ELN 49 83 53 87 / 0 0 0 0 RDM 40 78 46 83 / 0 0 0 0 LGD 44 76 49 81 / 0 0 0 0 GCD 43 77 47 81 / 0 0 0 0 DLS 53 86 57 89 / 0 0 0 0 && .PDT WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. && $$ EVENING UPDATE...83 SHORT TERM...77 LONG TERM....78 AVIATION...80